The rotor that provides a rotorcraft with lift, and possibly also with propulsion in a helicopter, is made up of a plurality of blades.
Once they have been fabricated, it is found that blades present differences in terms of mass and in terms of aerodynamic characteristics. Manufacturers therefore ensure that the various fabrication tolerances are as small as possible, but differences nevertheless remain between one blade and another.
It is therefore necessary to make provision for making adjustments, known as “balancing” by the person skilled in the art, so that all of the blades present the same characteristics.
Firstly, each blade must be capable of generating the same lift at a given pitch angle.
Consequently, the blades are provided with small control surfaces that are located on their trailing edges close to their free ends, and that are known as “tabs” to the person skilled in the art. Any change to the setting of such a tab relative to the airfoil of the blade generates a force that modifies the lift of the blade.
By adjusting the setting of tabs dedicated to this function, it is possible to achieve aerodynamic balancing of the blades.
In principle, this adjustment is performed by moving each tab manually through an appropriate angle, which angle is possibly different for each of the blades. To do this, each tab is generally constituted by a metal sheet that is secured to the blade for balancing.
Nevertheless, with reference to document U.S. Pat. No. 6,322,324, it is also possible to provide actuators for controlling the inclination of blade tabs. However, it can be difficult to convey electrical power supply means or actuator control means through the rotor and the blade.
Secondly, the blades need to have the same static moment, where static moment is defined as the product of the mass of the blade multiplied by the distance between the center of gravity of the blade and the axis of rotation of the rotor.
The manufacturer thus performs static balancing by mounting heavy elements on a blade. These heavy elements modify both the mass of the blade and the position of its center of gravity, thereby modifying its static moment.
Finally, and thirdly, it is necessary to balance the blades dynamically so that they induce identical twisting moments.
To perform dynamic balancing, it is appropriate to ensure from one blade to another that the distance between the center of gravity and the thrust center of the blade is the same.
A first type of system, described in particular in document U.S. Pat. No. 4,239,456, consists in dynamically balancing a blade with the help of flyweights suspended from the blade root.
Nevertheless, given the complexity of that first type of device and its repercussions on the aerodynamic efficiency of the blade, it is more usual to use a second type of device that seeks to cause heavy elements that are inserted at the tip of the blade to move along the chord of the blade.
Document EP 0 237 772 describes a balancing system comprising a cavity that is closed by an access hatch, the cavity receiving heavy elements in a given distribution along a chord.
Document DE 4 036 708 provides for inserting blocks made up of small plates that are secured to one another by a nut in an orifice of the blade that is closed by an access hatch. Balancing is performed by adjusting the number of plates.
Similarly, document FR 2 781 196 provides for inserting heavy elements in a housing that is open to the outside in order to perform dynamic balancing while conserving a static adjustment. When the heavy elements have been placed in their housing, it is closed by means of an access hatch.
The blade described in document FR 2 315 433 has three orifices in which stacks of heavy elements are arranged.
Each stack has a threaded rod passing therethrough so that the heavy elements are held in position by a self-locking nut screwed onto the rod.
Document DE 1 955 061 provides a housing formed at the tip of a blade, two heavy assemblies spaced apart by a spring being arranged in the housing. The housing is then closed by a cover.
The heavy assemblies have a threaded rod passing through them which rod projects from the housing by passing through the cover.
By turning the threaded rod, an operator moves the assemblies along the chord of the blade in order to balance the blade.
Thus, in systems of the second type, by modifying the locations of the heavy elements in an orifice, or by modifying their masses, an operator can balance the blade.
Nevertheless, balancing is constricting insofar as it often involves a large number of operations beginning with opening/closing the access hatch of the housing and continuing with manipulation of the heavy elements.
Furthermore, it is found that systems of the second type all open to the outside of a blade. The presence of closure hatches or covers is then not sufficient to avoid corrosion occurring on the various parts making up the system concerned.
Finally, devices of the second type can lead to problems of safety in the event of an access hatch opening in untimely manner, with the heavy elements possibly being ejected from the blade.